Which type of subwoofer unit is better among these? Plz see inside and suggest!?

If you live in a flat, downward firing probably your neighbours won't appreciate too much cause it sends the vibrations to them from their ceiling above. The front forward firing is one which is quite popular for example Bowers and Wilkins ASW600 which I have. The Yamaha subwoofer fires downwards and my neighbour has one which sounds good too. Obviously the larger the diameter, they have bigger coils which in turn generate lower htz. A good subwoofer is normally made from wood and braced and extremely heavy. The connection is via a special subwoofer cable and I use a Monster cable. The only three I would recommend are B&W, Velodyne or Paradigm but they are like over a $1000. So save hard !

Front firing subs are the best from my experience. If you have the room for a 15" powered sub, that would be ideal. In other words, the larger the better for bass power and the deepest notes. My favorite choice (and I own one myself) is the Dayton 15" powered subwoofer kit. It's easy to assemble, and for under $300 shipped, it can't be beat for a new powered sub performance. Search Amazon.com, PartsExpress.com, or eBay.com for the best deals. Happy listening!

it must be better to have right and left

Down-firing or front-firing subwoofers are generally about the same in performance. People have preferences based on their subjective experience with certain subs. Bigger woofers can deliver deeper and more powerful bass, but not necessarily as accurate as smaller woofers. The gap for this observation is usually from 10-12" woofers to 15" woofers. Of course, don't let that scare you away from a 15" woofer, most audiophiles typically agree that bigger is better. As far as design specs, a sealed sub will give you tighter and cleaner bass than a ported sub. Ported subs have their benefits though and can deliver louder output. Each sub has its benefits and tradeoffs in design. What sub would I recommend? It depends on your budget. I recently picked up a HSU STF-2 and am extremely pleased with its performance. I've tried a lot of subs below 400 and got nothing but featureless boom, without articulation or tightness. With the STF-2, I can notice the difference between different bass hits and impacts, bass has definition and character, its a huge difference. My recommendation is to figure out your budget and make a post on avsforum.com to get some feedback. You'll get a lot of options to choose from. If you want skip past all the weak subs and get straight to a proven winner, get a STF-2, its on sale now for 319 at www.hsuresearch.com. Hope this helps, good luck.